1. Field of the Invention
This invention relates to an ink jet recording apparatus for effecting recording with ink caused to adhere to a recording medium.
2. Related Background Art
Ink jet recording apparatuses in which ink is discharged from a recording head and caused to adhere to a recording medium to thereby effect the recording of information such as characters and images are known.
Apparatuses of this kind use chiefly paper or plastic sheets as a recording medium, and particularly are small in operation noise as compared with the other recording systems and are simple and inexpensive in their basic mechanical construction, and have been widely adopted as various recording output apparatuses of computers, word processors, etc.
In the apparatuses of this kind, however, a change in the discharge state of ink immediately appears in the form of the disturbance of recorded images and therefore, to effect image recording stably, it is important to stabilize the discharge state of ink. Now, the discharge state of ink depends greatly on the viscosity of the ink in the discharge port forming portion (hereinafter referred to as the discharge ports) of a recording head, and the production process and maximum volume of a bubble produced by a phase change in the ink when heat energy is applied to the ink. Further, the change in the viscosity of the ink depends on the time of the non-recording operation and the temperature of the ink, and the production process and maximum volume of the bubble also depend chiefly on the temperature of the ink and therefore, to finally stabilize the discharge state of the ink, it becomes important to control the temperature of the ink.
So, the applicant has proposed, as a system for uniformizing the temperature of the recording head and controlling the temperature of the ink, to mount a heat pipe on a side of the recording head, and effect the heating or heat radiation of the recording head through the heat pipe in accordance with the temperature of the recording head. Thereby it has been possible to effect the uniform temperature control of particularly the whole area of the so-called full line type recording head in which discharge ports are arranged along the recording width of recording paper.
FIG. 1 of the accompanying drawings shows an example of the prior art. In FIG. 1, the reference numeral 227 designates a recording medium which is conveyed in the direction of arrow A by conveying means, not shown. A recording head 221 is joined to a heat pipe 205 along the direction of arrangement of electro-thermal conversion members, and ink discharge ports, not shown, faced in a direction opposed to the recording medium 227. Ink is discharged from the discharge ports to the recording medium 227 in conformity with an image signal, whereby an image is recorded on the recording medium. The reference numeral 209 denotes a fan for blowing wind to fins 206 in accordance with the command of a control circuit 211. A temperature sensor 210 is mounted between the vicinity of the center of the heat pipe 205 and a support member 201 to detect temperature. The control circuit 211 is connected to the temperature sensor 210 and receives the temperature detected by the temperature sensor 210. The control circuit 211 is also connected to a heater 207 and the fan 209, and adjusts the input electric power to the heater 207 and the fan 209 or the ON-OFF time interval in accordance with the temperature detected by the temperature sensor 210.
However, the heat pipe is limited in its heat transporting ability, which depends on the size of the heat pipe. Relative to the heat transporting ability of the heat pipe mounted on the full line type recording head, the amount of heat produced from the recording head may become very great in some cases, depending on the kind of recorded image. That is, in some cases, the amount of heat generated by the recording head is over the maximum heat transport amount of the heat pipe. Particularly, where the recording head and heat radiation means attached to the heat pipe for radiating the heat of the recording head through the heat pipe are located at the same height, the evaporation speed of working fluid present on the inner surface of the heat pipe becomes great and the circulation of the working fluid becomes useless and there may be caused a phenomenon that the layer of the working fluid is divided into sections in the overheated portion thereof. This phenomenon is called dry-out, and when the working fluid is locally divided into sections, the circulation of the working fluid will no longer take place and the accumulation of heat will occur.
If the accumulation of heat occurs like this, the temperature of the recording head will rise and the non-uniformity of the temperature will occur in the lengthwise direction and thus, the temperature of the ink will become irregular and normal ink discharge will no longer take place, and density irregularity will occur to make it impossible to keep recorded images uniform and accomplish recording of high quality.